Include AuthenticationHelpers in Spree's Controllers

I was reviewing this article today https://gist.github.com/huoxito/5977700 and could verify the issue. A custom auth set up would definitely need this method https://github.com/spree/spree/blob/1afba5c199fd9817bcc233f11f77515ef78c9a33/core/lib/generators/spree/custom_user/templates/authentication_helpers.rb.tt#L10-12 since we removed the token from api calls on backend.

thanks for bringing this up @ffdd , could you please drop the other line? This is the only change we really need here:

```ruby
Spree::Api::BaseController.send :include, Spree::AuthenticationHelpers
```
api doesn't need all of those but I guess thats fine for now. I'll update that custom auth guide later.